It is common practice to treat sludge for pathogens by heating the sludge and holding the heated sludge in one or more reactors for a certain time period. Typically incoming sludge is heated to approximately 70.degree. C. and held in one or more reactors for approximately one hour. After being held in the reactor or reactors, the sludge is cooled and directed into a digester where the sludge is held for a selected time period. Further, in treating sludge for pathogens, it is known to heat the incoming sludge and cool the treated sludge by a network of heat exchangers. Essentially the heat exchangers remove heat from the treated sludge and add heat to the incoming sludge.
Cooling the treated sludge to a selected temperature is quite important. This is because the treated sludge is directed to a digester that is held at a certain temperature and if the treated sludge is of a temperature that substantially departs from the design temperature of the digester, then this can substantially and adversely affect the digestion process. Usually, in the case of a mesophillic digester, the sludge held therein is maintained at a temperature of approximately 35.degree. C. To avoid adversely impacting the sludge held in the digester, it is common practice to attempt to cool the treated sludge leaving the reactors to a target temperature range of approximately 34.degree.-38.degree. C.
In most pathogen reduction systems, it is virtually impossible to precisely control the temperature of the treated sludge being directed to the digester. In some cases, the heat exchanger network utilized relies on sludge-to-sludge heat exchangers, that is, the treated sludge leaving the reactor or reactors is circulated through a heat exchanger through which the incoming sludge passes. Thus the degree of heat transfer that takes place is dependent in large part on the flow rates of the incoming sludge and the treated sludge. Suffice it to say that there is very little practical opportunity to precisely control the temperature of the treated sludge, especially in cold climates. As a result, many pathogen reduction systems produce a treated sludge that, although subject to passing through one or more heat exchangers, fails to meet the target temperature range prior to introduction into a digester. Indeed, the problem is significant because of the lack of control of the sludge and/or heat transfer medium passing through the heat exchangers and the fact that the systems are designed for constant flow rates for all seasons of the year. Thus while it may be easy to meet the targeted temperature range in summer months for a given system, it may be quite difficult to meet the targeted temperature range using that same system during winter months.
Furthermore, in sludge treatment systems the heat exchangers tend to accumulate a build up of material such as grease foulings and other deposits within their inner walls as a result of the continuous passing of sludge through the heat exchangers. Eventually the heat exchangers must be cleaned. Typically this requires that the sludge treatment process be halted in order that the individual heat exchangers can be cleaned. The cleaning process for heat exchangers is time consuming and laborious. Not only is it costly in terms of labor to clean heat exchangers, but the fact that the sludge treatment process has to be shut down results in an even greater cost.